This invention relates generally to a method and apparatus for digital imaging devices and more specifically to compression of binary data representing images or pictures.
Prior art digital imaging systems like the PIC system of Wang Laboratories, Inc. are used to digitize pictures, charts, graphs and other images. In the imaging process the specimen is scanned for brightness and converted to a pattern of light and dark picture elements (pixels).
The pixels are created by comparing the specimen with threshold values and determining the status of the pixel based on whether the brightness of the location in question exceeds the threshold value.
Dithering is an image processing technique which creates a two level picture that gives the illusion of a grey scale (multilevel) picture by appropriately controlling the spatial density of light and dark pixels. The dithering is employed by applying a dither algorithm which consists of a pattern of threshold values in the form of a matrix, varying from the darkest to the brightest to a continuous tone or multilevel picture. The brightness of each pixel is compared with the elements in the dithering matrix, and thresholded accordingly, thus reducing the test specimen to a series of light and dark pixels which are represented by binary numbers.
Because of the large number of pixels required to make an acceptable image, a method of data compression is often used, called run-length encoding. Here, strings of identical binary bits are stored in memory as a code to reduce the number of bits in memory.
However, it has been discovered that run-length encoding works poorly when the continuous tone, or gray scale specimen contains large areas having a high number of alternating light and dark regions all of varying brightness. In these originals, there are no long strings of identical bits which may be encoded to achieve a compression of the image data, i.e., a reduction of the number of bits necessary to represent the image or picture.